x86/div64: Add a micro-optimization shortcut if base is power of two

In the target code I have a do_div(x, PAGE_SIZE). The x86-64
version of it was doing a shift and a mask which is clever. The
32bit version of it had a div operation in it which made me
think. After digging I noticed that x86 has an optimized version
of it. This patch adds this shift and mask optimization if base
is constant so we don't have any runtime "checking" overhead
since most users use a power of ten.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1322649814-544-1-git-send-email-bigeasy@linutronix.de
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/arch/x86/include/asm/div64.h b/arch/x86/include/asm/div64.h
index 9a2d644c08efc0981dbc13b1700e81917fef7c63..ced283ac79dfff3ca6c580da509136845ed1ce5b 100644 (file)
--- a/arch/x86/include/asm/div64.h
+++ b/arch/x86/include/asm/div64.h
@@ -4,6 +4,7 @@
 #ifdef CONFIG_X86_32
 
 #include <linux/types.h>
+#include <linux/log2.h>
 
 /*
  * do_div() is NOT a C function. It wants to return
@@ -21,15 +22,20 @@
 ({                                                             \
        unsigned long __upper, __low, __high, __mod, __base;    \
        __base = (base);                                        \
-       asm("":"=a" (__low), "=d" (__high) : "A" (n));          \
-       __upper = __high;                                       \
-       if (__high) {                                           \
-               __upper = __high % (__base);                    \
-               __high = __high / (__base);                     \
+       if (__builtin_constant_p(__base) && is_power_of_2(__base)) { \
+               __mod = n & (__base - 1);                       \
+               n >>= ilog2(__base);                            \
+       } else {                                                \
+               asm("" : "=a" (__low), "=d" (__high) : "A" (n));\
+               __upper = __high;                               \
+               if (__high) {                                   \
+                       __upper = __high % (__base);            \
+                       __high = __high / (__base);             \
+               }                                               \
+               asm("divl %2" : "=a" (__low), "=d" (__mod)      \
+                       : "rm" (__base), "0" (__low), "1" (__upper));   \
+               asm("" : "=A" (n) : "a" (__low), "d" (__high)); \
        }                                                       \
-       asm("divl %2":"=a" (__low), "=d" (__mod)                \
-           : "rm" (__base), "0" (__low), "1" (__upper));       \
-       asm("":"=A" (n) : "a" (__low), "d" (__high));           \
        __mod;                                                  \
 })